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Thread: Preamp stage help please.

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    Preamp stage help please.

    Hi all. I've built or converted about 20 or so amps now and each one has basically had 2 or 2 1/2 gain stages. Mostly 2 gain stages with a blackface Fender style tone stack. I just started converting an old 1950's Bogen CHA-33 PA amp (schematic attached) to a guitar amp which has one 12AX7 (V1) one 6AV6 (V2) one 12AX7 (V3) with two 6L6's and two 5Y3 rectifiers.

    So far I've stripped out the tone stack and installed a 1 meg Vol pot, 250k treble and bass pots ala Fender. I also bumped up the preamp voltage to approx 190-200 volts on the 12AX7 plates/140 on the 6AV6 plate and that's it.

    I'd like to have 2 channels or at least 2 inputs and I'd like to put the tone stack either between the two triodes of V1 (12AX7) or after V1 and before the 6AV6 (V2) but I'd like to have one input run through 2 gain stages and the other input run through 2 1/2 or 3 gain stages (depending on what style phase inverter I use and if it takes 1/2 of a 12AXY or both triodes) but I dont know how to wire it up.

    There are 2 schematics attached since one may be easier to read than the other. They're both the same but I imagine that they were drawn at different times.

    I'd appreciate any help.
    Thanks.
    Rob.

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    Old Timer Leo_Gnardo's Avatar
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    Inquiring minds want to know: what do you mean by 1/2 a gain stage???

    Either you pass signal thru A stage, or you don't. It's sort of ummm, quantum. One or zero. Half isn't a choice. Well as far as I know, maybe we'll learn something from your answer. Thanx in advance!

    Sign me, puzzled.

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    Did you only want one tone stack but two channels?

    So how bout one-half AX7 each per channel (voiced as you like), then mix the channels like the original Bogen, put the tone stack after the channels are summed. Then use the AV6 as a second shared gain stage, like the one in an AB165 Bassman, then the standard "booster" to push a Cathodyne PI driving the 6L6s? I tried something similar in an old Thor head, but wit one channel. It was Friggin Amazeballs as a rock machine. Might be a little over the top, though I only had a single Tone knob & Volume in stead of a more lossy Tone "Stack" a la Fender...

    Basically each channel would have 3 gain stages, two of them shared... the main issue is, two gain stages & a Cathodyne without its driver may not be enough to push 6L6s, especially with a Fender stack in it... So figure either Cathodyne OR LTPI would effectively need two triodes to drive those big bottles, leaving 3 triode for two channels. The small Tweed Fenders are the most "triode-efficient" ones I know of.

    Justin

    Edit: you could also put the tone stack after the 6AV6, right before the PI "driver" stage...

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    Thanks Justin. A single tone stack is fine but I'd like one channel/input to have more gain than the other. I can also add a switch for the cathode bypass caps/NFB. Something like that but I need help wiring it up.

    How do I pass from one stage to the next but add another input at the same time? Thats where I start scratching my head.
    Thank you.

    Hi Leo. Some tubes have a single tube in the bottle and other tubes like a 12A*7 or 6EU7 have dual triodes which are basically 2 tubes in the same bottle so each half of the tube is considered 1/2 of a gain stage when placed in a preamp circuit.
    Hope that helps you some.
    Rob.

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    Hey Rob,

    Each twin triode is capable of being 2 gain stages. Each triode section is A gain stage, potentially. So if you use a whole 12AX7 in one channel like a Fender Normal channel, that is two gain stages. The Reverb channel would have 3. You almost have to think of an AX7 as 2 completely separate tubes, as that is indeed what they are. You could wire up only one triode in the tube (like a Fender Bassman head) and it is still considered one gain stage, not half.

    As far as how to wire the channels, see how the Bogen's Phono input bypasses the first tube & feeds the AV6 directly? You could do something like that... the main reason I was suggesting the Tweed way was because, for all intents and purposes, you have three gain stages to be used among two channels, after you eliminate one tube for the PI. Otherwise, you could use a single triode stage and bypass the tone stack, feeding it straight into the PI Driver... basically, move the wire from the 220K (R11) feeding into the AV6 and move it to feed V3A instead... that way, one channel has a straight Fender Normal T/B/Vkind of layout, and the other is like a 5F10 Harvard (I think) with just a Volume...

    Just thinking out loud...

    Justin

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    Lifetime Member Enzo's Avatar
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    yep, a stage is a stage, it doesn't matter how many of them are in a particular tube. A 6C10 has three triodes, compared to 12AX7 with two. It may sound trivial, but it matters when communicating that we all use the same terms. A two channel preamp might have two 12AX7s. I could wire them up as one 12AX7 serving two stages in one channel. Or I could wire them up as one side of each tube was one channel, and the remaining sides were the second channel.

    I think most everyone knows what a gain stage is, though many people use it to refer to any triode stage. To me a gain stage has gain, a cathode follower stage doesn't really have gain, so I don't use the term gain stage for those. Or at least I try not to.

    And one final nit. V numbers for tubes - V1, V2, V3, etc - refer to the schematic, not the chassis. In many cases, even most cases, the V numbers on the schematic are the same as the tube order across the chassis. But not always. Myself, I'd organize my drawings so the numbers agreed with tube position. SO if the schematic calls a tube V4, do not assume it is the fourth tube over until you determine if that is true. Look at the PV 5150/6505 amps as examples.

    I'll yak about my other thing - misuse of "comprise" - somewhere else.

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    Quote Originally Posted by Justin Thomas View Post
    As far as how to wire the channels, see how the Bogen's Phono input bypasses the first tube & feeds the AV6 directly? You could do something like that... the main reason I was suggesting the Tweed way was because, for all intents and purposes, you have three gain stages to be used among two channels, after you eliminate one tube for the PI.
    Justin
    Actually I'll have 4 gain stages total if I us a a split load/cathodyne PI. I can run input 2 just like the phono input was wired on the schematic and it still goes through the tone stack. So, I'd have 2 gain stages on one channel and up to 4 gain stages on the other input. 4 would be a bit much but 3 would probably work ok with a blackface Vol/Treble/Bass tone stack.
    Do you see any issues with Justin?

    Thanks!

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    Quote Originally Posted by Enzo View Post

    And one final nit. V numbers for tubes - V1, V2, V3, etc - refer to the schematic, not the chassis.
    The Bogen CHA33 I'm working on right now is a perfect example Enzo. The 6AV6 is first valve in line then come the two 12AX7's.

    It can get confusing when all of the valves are the same. Thats when I have to get the eraser (solder sucker) out

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    Another question if you guys dont mind. I cant tell from the drawing if the plate of the first triode is going to the grid of the second triode of the PI or not.
    Is it a Paraphase or chathodyne PI????
    Thanks

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    In your channel with only 2 gain stages, a circuit of Stage 1 => Tone Stack&/or Volume => Stage 2 => Cathodyne => 6L6s May not be enough to drive the output tubes to your satisfaction... if you were driving EL84s or 6V6s, maybe. It's n or that it wouldn't "work," I just wonder if the results would be disappointing in that channel. You really lose a lot in that tone stack, compared to if you used a single Tone control like the Harvard. I may be entirely wrong - I'm basing my experience on my one big-bottle build...

    Justin

    PS it's not a Cathodyne...

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    @ Enzo:

    A 12AX7 comprises 2 separate triodes?

    Justin

    I had to look it up. Though I didn't see where it was used in the thread?

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    Last edited by Justin Thomas; 02-05-2017 at 08:30 AM.
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Yes, thank you for understanding. I even have seen "comprised of" in such publications as Scientific American and National Geographic, though not often.

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    Bent Member Chuck H's Avatar
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    Justin has a good idea, but there's a potential rub (no offense brother). The block diagram below outlines something like the idea. As you'll see there is no good way to incorporate fully independent volume control. In fact there's only one way and that would be to place the volume control for the lower gain channel between the input jack and that channels first gain stage. This has historically proven to be tonally undesirable. It also has the drawback of needing a large-ish value "mix" resistor after the higher gain channel volume to avoid control interaction and that would attenuate the higher gain channel considerably. The idea also has the drawback of floating the higher gain channel input stage when the lower gain channel is selected. Since you'll need to keep the second gain stage in use for the higher gain channel you can't use a grounding switch scheme. That will mean some extra high gain hiss riding on the lower gain channel when it's selected. That said...

    If independent volume control isn't critical then you could put a second volume control at any of the other locations and it should be fine. Just a little noisier than necessary and without independent channel control for the higher gain channel. Mesa actually did some similar designs and people liked them just fine.

    I came up with an idea some time ago and this seems like a good application for it. It also doesn't give you independent control, but it offers less noise, simpler switching and one less jack to wire. See the schematic below.

    You would use all three available triodes (the concertina PI isn't a gain stage) for both the low and high gain "channels". Using this circuit on the first gain stage will provide more gain through the overall circuit. Using it on the second stage should still provide plenty of gain and would allow you to fully bypass the first gain stage. Which IME makes for lower hum noise. The 10k fixed cathode resistance should keep things pretty clean for the lower gain channel. The circuit provides a 17dB boost down to the knee of the bypass cap. Because this is a boost you probably wouldn't want to fully bypass the switched cathode or the clipped tone could have too much LF. I indicated probable useful values to try in the schem. The 25k pot would be on the control panel as a boost level control with a range from maybe 3dB up to the full 17dB. The impedance of the cathode circuit should be low enough to avoid any trouble with noise or capacitance in the footswitch cable. Sort of like channel switching without the logic devices or relays
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    Last edited by Chuck H; 02-05-2017 at 09:17 PM.
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    Chuck,

    No offense taken here! Ain't the first time I learnt from you, prolly won't be the last!

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    Chuck,

    No offense taken here! Ain't the first time I learnt from you, prolly won't be the last!

    Justin
    I've learned plenty from everyone here. Including you. And anyway, Mesa was selling amps with designs like your idea when we still had long hair

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    Thank you very much Justin. Much appreciated.

    I just finished a single input 6L6 conversion with 2 gain stages (one 12AX7 for gain and half of the second 12AX7 for the PI leaving 1/2 of a triode unused) and with 5881's, while it doesn't scream dirty it does get a nice bluesy crunch but only at very high volume.

    I think another 1/2 gain stage would have been perfect.
    Is there any way to use the unused 1/2 of the 12AX7 as a gain stage? I used the AA964 Princeton schematic as a template (no tremolo on my build but with adjustable grid bias) which I've attached a link to below.
    Rob.

    Princeton Schematic
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    Quote Originally Posted by Chuck H View Post

    If independent volume control isn't critical then you could put a second volume control at any of the other locations and it should be fine. Just a little noisier than necessary and without independent channel control for the higher gain channel.
    Independent volume controls would be nice Chuck but either way would be ok as long as I preserve the blackface tone stack sound.
    Has the circuit that you've proposed been tested? I really like the idea of the boost level control.
    Thanks for the incredibly detailed response.
    Rob.

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    I'm more of a "tack it together & see" guy, but, if you take out the reverb & tremolo on an AA1164 Princeton Reverb... might need to use a voltage divider between the second and third stages (which in the PR seems to be the 3.3MR/Reverb circuit) and tweak some values. But, it's a starting point.

    A lot of the multi-gain-stage uber-distortion amps use the voltage dividers to keep the next stage from getting slammed with such a large signal that the end result is "AC/DC with Volume on 1, Metallica on 5, and Sonic Mayhem on 10!" It happened to that "Thorange" I made, having a gain stage, Volume, Tone, gain stage, gain stage, PI - no attenuation... Completely awesome, completely out of control, almost completely useless for anything less than Ted Nugent. But your results may differ, because of a more complex and lossy Tone stack.

    As I said, I'm not entirely sure what I'm doing, but... you won't kill anything trying! Except your hearing. Maybe. And Chuck has probably already tried just about anything I mention, so... I get the impression we tend to bark up the same trees.

    Justin

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    Bent Member Chuck H's Avatar
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    Well the circuit I posted has only been tested in my mind... But it works. The level control would sort of be the second volume control as I see it. You'll have the first volume control for the nice channel and the boost level would be the only additional gain control when selecting the naughty channel. I would surely use it on the second gain stage to overdrive the third. The TW circuit has only a single load resistor of 150k between the second and third stage and most TW users report that the useful gain adjustment peaks at one o'clock on the volume. So Justin's on to this one. You may need to attenuate between stage two and three.

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    Quote Originally Posted by Chuck H View Post
    Well the circuit I posted has only been tested in my mind... But it works.
    Sounds like a great plan Chuck and I'd really like to use it but first I need to solve another issue.... It never ends.

    I replaced all of the old wax style coupling caps last night and I wanted to test it and see how it sounded with 2 gain stages or possibly 2 1/2 gain stages because I still cant figure out what type of phase inverter it has so one triode of the last 12AX7 (V3) may be used for gain. I'm not sure.

    I'm running only input (1 meg grid leak and a 33k grid stopper) from V1 (12ax7) to V3 (12ax7) to the output stage. The 6AV6 (V2) is totally disconnected except for the heaters.

    I get no audio unless I touch my DC volts probe to grid A (pin 7) of V3. Then the audio kicks in but the volume fluctuates very slowly. If I knew more about the Princeton style tremolo I might be able to figure out the problem.

    I bumped up the preamp plate feed voltage so they now read higher than shown on the schematic which is attached in my first post.
    Voltages off the top of my head are as follows.
    V1 (12ax7) approx 190v on both plates. 1.4v on the cathodes.

    V3 12ax7 (either PI or PI and 1/2 gain stage) pin 1=170v. Pin 7 = 310v <--- Should match pin 1?
    V3 cathodes measure pin 3 1.5v. Pin 3 1.1v

    V4 & V5 (6L6) = 440v plates/320v screens. Screens seem a bit low.
    20v drop across the cathode bias resistor.

    I need to figure out what type of phase inverter this amp has so I'll know whether or not both triodes of V3 are properly biased.

    Thanks all.
    Rob.
    .

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    Here are a couple of pictures for whatever it's worth. Very difficult to see whats going on with point to point.

    The tone stack is from a Princeton AA964. Some components are jumpered together and tacked in.
    One of the jumpers is going from the input to V1 grid (pin 7) to the middle lug of the volume control. The other jumper is V1 plate (pin 6) to V3 grid (pin 7) The mid range resistor (bass pot left lug to ground)is tacked in temporarily as I may add a mid range control.
    EDIT. I forgot to mention. I measured all of the filter caps with my ESR meter and while one is border line they're all within spec.
    Thanks guys.
    Rob.

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    Quote Originally Posted by Stratz View Post
    I need to figure out what type of phase inverter this amp has so I'll know whether or not both triodes of V3 are properly bias ed.
    .
    It's a Paraphase. Not exactly sure which specific "kind," because there are several. But, it's clearly not an LTPI or Cathodyne. If it were a Cathodyne, one of the power tubes would be fed by a plate, the other by a cathode. If you follow the grids of the 6L6s BACKWARDS from their .033 coupling caps, you see each cap fed by a plate. R24, 25, 28 are a series of voltage dividers which take the amplified signal from the plate of V3A and take it down to the same or similar l level as the signal feeding the grid of V3A. Then that triode (V3B) amplifies it to the same level as what's coming from the plate of V3A, but out of phase with V3A. The main trick with these is balancing the two...

    Hope that helps! Funny enough, it's about the only PI I feel I have even a remote grasp on!

    Justin

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    Thank you Justin. If I change it to a cathodyne will I be able to use one triode for a gain stage?
    Phase inverters are a weak point for me so I'll need some hand holding if I can use the entire tube.

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    Yup, just like a Princeton.

    I guess I personally don't think of the "triode before the Cathodyne" as a "gain stage." I consider it as a part of the PI. I see its job as "drive the PI enough so the PI can drive the power tubes suitably." I guess it's technically a gain stage. It looks exactly like the others, so...

    I would be curious, because the Paraphase uses a gain stage to drive one power tube, and another out-of-phase gain stage to drive the other. So basically, there's a gain stage before each power tube. The Cathodyne has a gain of 0.9 or something, so it needs a single gain stage before it to get enough drive. So it seems it may be kind of equivalent to me - a gain stage driving each power tube.

    Where the difference comes in is how the two halves are balanced, how the circuit distorts under regular & overdrive conditions, and other such phenomena. Probably no more "gain" in one over the other, but the CHARACTER of the sound is where it counts.

    The LTPI can also provide gain in and of itself, it's just how that one works. Personally, I enjoy amps with Cathodynes - there's a certain crispness right at the edges of overdrive. I've never played a Paraphase amp, other than Ampegs, and they were meant to never distort anyway, so I don't use them to judge.

    Justin

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    I absolutely understand your point and I'd be thrilled to leave the LTP right as it is but I believe my volume swell problem is in the PI. Unfortunately I am much less familiar (read "not a clue") with the LTP as I am with the cathodyne.

    If you look at this schematic you'll see that the cathodyne only requires one triode of a 12AX7. Even though the other triode is being used for the tremolo circuit you can get away with only 1/2 a 12AX7 and just leave the other 1/2 un-wired..

    I'm very new to building amplifiers. I started with a Mojo Deluxe Reverb kit last summer. Built a 5e3 Tweed kit and an 18 Watt Marshall from a kit and I really didnt learn much about amps at all because it's pretty much "paint by numbers" when you follow a layout. So I started to convert old PA's and Hi-Fi's to guitar amps and not only did I learn about 1000X more than any kit build I also taught myself how to read a schematic.

    I've done a bunch of conversions and they sound really, really great. I did a 6L6 conversion and I made a LTP PI and I had nothing but problems so I guess I'm a bit intimidated by the LTP. The other problem is that I still dont know how to use the left over triode from a cathodyne PI for a gain stage so I'm equally stuck with the cathodyne and LTP if you get my drift,

    I'm hoping someone will chime in and explain how to use the left over triode from a cathodyne as a gain stage. It's a shame to leave 1/2 a tube on the table so to speak.
    Rob.

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    Lifetime Member Enzo's Avatar
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    You get no audio passing unless you put a meter probe on a grid? That sounds to me like that grid has nio return resistor. Powr off, what resistance to ground from that pin 7?

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    Quote Originally Posted by Enzo View Post
    You get no audio passing unless you put a meter probe on a grid? That sounds to me like that grid has nio return resistor. Powr off, what resistance to ground from that pin 7?
    I get exactly 0 ohms sir.
    Sounds like you're on the right track Enzo I had ran a jumper (just testing) from a .1 cap off the V3 pin 7 grid to the plate of the previous gain stage. I dont usually have to use grid resistors on Fender style preamp's so the grids must be seeing resistance from somewhere down the line.

    What do I need to do to correct this?
    Thanks!

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    Bent Member Chuck H's Avatar
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    Are you sure you got zero and not infinity? It should be pretty plain to see if that pin 7 is wired directly to ground.?.

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    You can't let grids float.

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    Quote Originally Posted by Chuck H View Post
    Are you sure you got zero and not infinity? It should be pretty plain to see if that pin 7 is wired directly to ground.?.
    Sorry Chuck. I should have wrote infinity or OL. I had no continuity.

    I hooked up my decade box and a 33kR seemed the best match between noisy and lower output. The tube is also properly biased now.
    I can continue on with the modified preamp section now that I know the PI and output stage is wired properly.
    Thanks
    Rob.

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    Quote Originally Posted by Enzo View Post
    You can't let grids float.
    Thanks very much for pointing me in the right direction Enzo.

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    Stratz,
    Not sure how much "re-wiring" you might contemplate. But if you want 2 channels with different gain structure then might I suggest a bit of thinking outside the box and replace the 6AV6 triode in the 7 pin socket with a 6AU6 pentode.
    I have "re-purposed" a 7 pin 6AV6 socket in a previous conversion but in that case I used the 6AU6 as a tremelo oscillator (like some old Danelectro).
    Cheers,
    Ian

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    Thanks Ian. I have quite a few NOS 6AU6's laying around.

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